Method of treating wood slats



United States Patent 9 METHOD OF TREATING WOOD SLATS Edwin A. Smith, Stockton, Calif.

Application September 21, 1954, Serial No. 457,452

Claims. (Cl. 14428) The invention relates to an improved process for the treatment of wood slats and especially to an improved.

method of treating wood slats of the variety used in the manufacture of wood pencils.

Present day methods of drying lumber, and especially lumber of small sizes such as short slats, are in general quite unsatisfactory. Not only does the drying period consume an inordinate amount of time but owing to the process used, an excessive number of defects are produced in the slats, which requires either downgrading or outright rejection.

A present day process of drying slats is well exemplified in the method used in the manufacture of wood slats made, for example, of California incense cedar and which are used in the production of wooden pencils, the pencil manufacturer buying the treated slats and converting them into pencils by the customary steps of grooving, inserting the leads, gluing and shaping. The slats are approximately 185 mm. (7.25 in.) in length, 65 mm. (2.50 in.) in width and 5 mm. (0.20 in.) in thickness, and are cut by gang arrangements of cutting saws from air-dried boards or from timber which ordinarily is quite green, that is to say, with a high moisture content. After sorting and inspection, a thin coat of wax is frequently applied to the faces of the slats to increase cutability of the wood during pencil manufacture and ultimate use by the consumer.

The slats are then arranged, under heretofore used methods, into stacks or bundles, each bundle containing some 264 slats. In a bundle, it is customary to place 4 rows of slats edge to edge, for example, and then to pile up slats on each row so that each row contains approximately 66 slats, the completed stack or bundle thereby forming a compact mass having the dimensions 185 mm. x 260 mm. x 330 mm. (approximately 7.25 in. x in. x 13 in.) and with the slat ends exposed. A cord or other binding is thereupon applied to the stack to cause it to hold its shape.

After stacking, the slat bundles are loaded on trucks or carts, under methods previously used, and, if the moisture content of the slats is in excess of a predetermined amount, such as 50%, the bundles are subjected to drying, the carts being moved to the drying areas. Some operators utilize kilns to effect drying whereas others prefer to air-dry, i. e., to dry the wood by placing the material out-of-doors in a large area termed a drying yard. Owing to the limitations inherent in the previously used methods of drying slats, kiln drying frequently consumes from 8 to 9 days for lumber of high moisture content whereas air-drying might well and often does consume as long as 3 to 4 weeks or more, resulting in either case, in a large and expensive in-process inventory.

At the conclusion of the drying period, the slat bundles are carted to and immersed in a tank filled with pencil slat stain and therein subjected to pressure in order to drive the stain into the wood and bring all of the stainimpregnated slats to approximately the same color.

It is thereafter necessary to remove the slats from the stain tank and again subject the stacks to a further drying period. Usually, the after-stain drying is performed in a kiln, this kiln drying period taking from 16 to 23 days to bring the moisture content down to the desired amount.

After removal from the kiln, the stacks are trucked to a sorting station and the stacks broken apart to permit the workers to inspect the individual slats and to test them to see that requirements are met. The slats passing established standards are then packed and carted away for storage and subsequent shipping.

It can be seen that the foregoing process, now used, is not only time consuming and, where air-drying is utilized, requires a large amount of storage area, but it also entails an inordinate amount of handling and re-handling, all of which increases costs to a high figure. Furthermore, the heretofore-used process results in a seriously high rate of rejects owing to the creation of defects in the slats such as end cracks, end checks, splits, etc., caused by improper and uneven drying brought about by having to dry to approximately the same moisture content slats on and near the outside of the bundle and those on the inside. In order to dry the interior slats, the outer slats have heretofore been subjected to excessive drying conditions, with attendant creation of defects.

It is therefore an object of the present invention to provide a method of treating wood slats which reduces the number of wood slats which have to be rejected owing to defects arising in the course of the treating operation.

It is another object of the invention to provide a method of treating wood slats which markedly reduces the number of separate transportation and tore employed.

It is a still further object of the invention to provide a method of treating Wood slats which cuts down the need for large storage areas heretofore utilized during airdrying, and which reduces the amount of slat in-process inventory heretofore necessary owing to the large number of slats located in the kilns and air-drying yards.

It is yet another object of the invention to provide a generally improved method of treating Wood slats.

Other objects together with the foregoing are obtained in the practice of the exemplified method hereinafter shown and described.

Figure 1 is a perspective view of a portion of five typical slat rows, with the slats arranged in accordance with the slat treating method of my invention.

Figure 2 is a diagrammatic showing of the various stations utilized in the exemplified method and the flow of material between stations.

Figure 3 is a perspective semi-diagrammatic view of a drying kiln used in the drying stations appearing in Figure 2, a portion of the figure being broken away to reduce the extent thereof.

Freshly cut wood always contains water, some of the water being in the cell cavities (free water) and the balance in the cell Walls (hygroscopic water). The amount of water in green lumber varies, on an ovendry weight basis, from to over 200% depending on various factors, such as the species, the location of the growing tree, the time of year when out and the section of the tree from which the wood was obtained, sap wood, for example, having more water than heart wood. In the case of the particular wood here exemplified, California incense cedar (Libocedrus decurrens), the average moisture content of green wood is 213% for the sapwood and for the heartwood.

Most of the water in green wood must be removed to render the wood suitable for its intended use, as, for example, the manufacture of pencils. water retained by the wood, and, conversely, the amount of water lost from the wood during drying, depends carting steps hereto-- The amount of upon the temperature and relative humidity of the par ticular surrounding medium, or atmosphere, to which the wood is subjected; as a corollary, the same factors of atmospheric temperature and relative humidity control the rate of moisture loss, i. e., the drying rate. When subjected to a given elevated temperature and to a given relative humidity, wood gives off moisture to the surrounding atmosphere until the moisture in the wood comes to a balance with the moisture in the atmosphere, the Wood moisture at such point being known as the equilibrium moisture content (E. M. C.).

During drying, the moisture moves from zones or layers, of higher, to zones of lower moisture content, the rate of moisture flow being dependent upon the steepness of the moisture gradient and the temperature of the wood. The free water in the cell cavities moves outwardly under force of capillary action, somewhat in the nature of fluid movement in a candle wick. When the point is obtained at which the cell cavity is emptied, the cell wall still being saturated, the fiber saturation point is reached. The moisture content at this point, for all Woods, is approximately 30%, and marks a critical stage, for, as drying proceeds below this moisture content, more heat is required (for, to drive the hygroscopic water from the cell wall takes more heat than that needed to empty the free water from the cavity), the wood cell for the first time commences to shrink, and physical and mechanical changes in the properties of the wood begin to take place. (Properties of Wood Related to Drying, U. S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, Wisconsin, June 1951.)

The hygroscopic or cell wall moisture moves from the cell walls largely by a process of diffusion, the rate of drying by diffusion being a function of the wood temperature and the moisture gradient between the wood surface and the interior zones.

It can therefore be seen that both in drying the wood down to the fiber saturation point, and in lowering the moisture content below that point, the most rapid rate of drying is obtained where the wood temperature is high (within safe limits depending on the particular wood) and the relative humidity is low, and especially where the low humidity atmosphere can be kept in contact with the surface of the wood, so as to create and maintain a high moisture gradient with respect to the interior zones. Most effective drying is obtained, therefore, by exposing the maximum possible amount of slat surface area to the low humidity-elevated temperature atmosphere, and by providing for free circulation of the atmosphere over the surfaces exposed.

With particular reference to Figure 1, it will be seen that in the method of treating wood slats of my invention, wood pencil slat stock being used for sake of example, a plurality of wood slats 6, each having been cut, approximately, to a length 7 of 7.25 inches, width 8, or height, of 2.50 inches, and edge thickness 9, of 0.20 inch, are disposed in a plurality of transverse rows 11, 12, 13, 14 and 15.

In each of the rows, the slats in the row are preferably aligned, with the adjacent ends 16 and 17 of the adjacent slats in the row, facing each other. The ends, however, are not in abutment. Instead, the ends are separated by space or distance 18 which, for the slats herein described, would be approximately 4% inches. This space between ends permits free circulation of the drying air, and also allows escape, from the exposed ends, of the considerable quantity of moisture leaving through the ends. Customarily, the slats are cut so that the slat sides 21 and 22, and the edges 23 and 24, respectively, have side-grain surfaces, while the ends 16 and 17 have end-grain surfaces, the cells of the wood being substantially parallel to the slat length. Since moisture diffusion toward the end-grain surface of a piece of wood is approximately to times faster than diffusion toward the edges and sides (having sidegrain surfaces), moisture loss through the ends in a slat of the length exemplified is substantial. It has therefore been found that by separating or spacing the ends of adjacent slats in the same row in the fashion described, the over-all drying rate is improved.

Even greater efficiency of drying is obtained by arranging adjacent transverse rows 11 and 12, for example, in staggered or offset relation. As most clearly appears in Figure l, the staggered arrangement is preferably made so that the corresponding ends 16 and 17 of the slats in alternate transverse rows 11, 13 and 15, for example, are all in longitudinal alignment, and the ends of the slats in the adjacent alternate transverse rows, 12 and 14, for example, are also in longitudinal alignment. With the slats in the attitude and relationship herein shown, not only the ends of the slats, but a large portion of the vertical side faces as well, are exposed, and act as effective moisture emitters to the surrounding atmosphere.

The top edges 23 and 24 of the slats likewise provide moisture emitting surfaces of considerable extent. So also, the corresponding lower or bottom edges 26 and 27, respectively, constitute efficient moisture escape surfaces since the slats are supported by a perforated surface 31 or screen, for example, which permits free circulation of atmosphere to and away from the bottom edge surfaces. Even greater bottom edge exposure is obtainable in the event a plurality of longitudinal chains, for example, were to be used instead of the perforated surface 31. An individual chain, in this case, would be located below each of the overlapping portions of the slats and adjacent the outer ends of the outermost slats.

Of especial interest is the plurality of open passageways or chambers 32 and 33, formed between the facing ends of adjacent slats in each row, and between the facing sides of the slats in the alternate rows. Free atmospheric circulation into and out of the passageways enhances drying of the adjacent wood surfaces. Treating the perforated supporting surface 31 or screen as substantially unobstructed so far as the circulation of atmosphere is concerned, and considering a typical interior slat, 34, for example, it can be seen that except for the portions of the side faces near the slat ends and which are overlapped by the adjacent slat end face portions, the slat 34 is substantially surrounded by the circulating drying atmosphere. As a consequence, each slat is permitted to dry substantially as if it were individually introduced into a drying kiln. Advantages, such as close control of atmosphere, wood temperature and moisture gradient, as well as uniform ultimate moisture content and rapid drying rate are therefore fully realized, an achievement practically impossible under the heretofore used methods in which the slats are bundled into a comr pact mass forming a maze of moisture traps and with the slats placed in such close face to face and edge to edge relation as to preclude uniform drying.

As was heretofore stated, as drying proceeds to a moisture content below the fiber saturation point (about 30% moisture content) the wood undergoes shrinkage. When the wood has dried to 15% moisture content, about onehalf the total possible shrinkage has taken place. Uniform shrinkage, resulting in a corresponding uniform slight reduction in over-all dimension is not particularly objectionable. But where wood is subjected to very severe drying stresses, it may fail along lines of weakness and cause such undesirable features as end checks and end splits, the ends of the wood being particularly susceptible to these downgrading or rejcctable characteristics.

The method of treating wood slats herein described comprises, therefore, not only the overlapping and offset arrangement of slat rows which effects an open slat latticework permitting free circulation of atmosphere attended by the advantages stated, but also an arrangement wherein the end portions of the side faces of the adjacent slats are overlapped by a predetermined amount so as to reduce end checks and end splits resulting from over-rapid drying. As appears most clearly in Figure 1, the optimum amount of overlap 41, for the exemplified slats has been determined by experimentation to be 1.25 inches. Referring, again to the interior slat 34, it will be seen that at both ends of the slat 34 the end portions 42 of the side faces 21 are overlapped, and that the overlapped side surfaces are therefore not open or free to the atmosphere but instead are in abutment with the adjacent end portions of the slat faces. The latter, in turn, are covered or sealed off by the end portions of the slat 34. Each slat, therefore, serves to cover four adjacent side face portions owing to the unique arrangement of the slats as herein shown and described.

Since a predetermined amount 41 of each of the side faces is covered by the adjacent slats, the moisture on the contiguous or abutting slat surfaces is not readily scavenged by the drying atmosphere. As a consequence, the covered surfaces remain at a high moisture content in relation to exposed surfaecs. Thus, very little moisture gradient is established between the overlapped surfaces and the overlapped interior zones, resulting in a reduced drying rate for the covered end zones. Similarly, the temperature of the overlapped surfaces and interior zones is relatively low, the heated atmosphere not so readily reaching these surfaces. The combined result is that the overlapped portions of the slat ends do not dry out nearly as rapidly as the exposed areas bounding the open chambers 32.

The amount 41 of overlap therefore reflects a desire to subject a maximum area to free circulation of the atmosphere, but it also takes cognizance of the important drying property of wood heretofore mentioned, viz., that flow toward the end-grain surfaces 16 and 17 is from to times the rate of flow toward the sides and edges of the slat. The overlap permits the ends to dry by moisture flow toward the end-grain surfaces but cuts down the rate of drying in the end zones by lowing the amount of moisture which overwise would escape through the sides. Since the overlap reduces the drying rate near the slat ends and a lower drying rate near the ends cuts down the incidence of end checks and end splits, it will be seen that a superior quality of product is obtained.

The predetermined amount of overlap herein described likewise obtains another desirable result, namely, that at the end of the drying period the moisture content is substantially uniform throughout the slat. This situation is realized since as the moisture content of a particular mass of wood is lowered, the drying rate drops off, as well, i. e., the wood becomes harder to dry. Thus, while at first the exposed central areas dry quickly, the rate of drying of the exposed areas falls off relative to the rate in the wetter overlapped end zones. Near the end of the drying period, therefore, it is found that the moisture content of the end zones closely approaches the moisture content in the uncovered portion. In the meantime, however, and during the initial drying stages when end checking, splitting and collapse of the cells are most prevalent, the end portions are protected against too rapid drying causing these defects.

Figure 2 illustrates the various stations comprising the method herein disclosed. The slat cutting station 51, and sorting station 52, are conventional, the material flow being indicated by the arrows 53. After sorting of the slots, and rejection of sub-standard or off-size material, a positioning station 54 is inserted in the production line, where the slats are arranged in the transverse, alternately offset rows as shown in Figure 1. The positioning is performed either by hand or by appropriate mechanism (not shown). In either event, a continuous longitudinal arrangement of transverse rows is obtained, the bottom edges of the slats being supported preferably by a moving surface so that the rows of slats will be transported away from the positioning station 54 and onwardly to a waxing station 56, if waxing is to be performed. At the waxing station, a daub of wax is applied to one or both of the side faces of the slats such as by hand or by a mechanical dauber (not shown).

From the waxing station the production line of slat rows proceeds onwardly to a first drying station 57 and there the slats are led onto the perforated screen surface 31, or onto a plurality of longitudinally disposed chains, each chain supporting the slats underneath the overlapped portions and under the outer slat ends as described above. For convenience, the endless screen 31 type of support is shown in Figure 3, the upper run of the screen supporting and carrying the slats through an entrance aperture 61 and exit aperture (not shown) in a kiln 62, the kiln being provided with suitable accessories (not shown) such as circulating fans, hot air and steam sources, and temperature and humidity regulating devices. For increasing production, the line may be reproduced to run in parallel fashion, Figure 3 illustrating, for example,

, duplicate drying lines 63 and 64, the two lines of slats both advancing into and out of the kiln in the direction indicated by the arrows 65 and 66, respectively.

The first drying station 57 is divided by interior partitions 68 and 69 into three drying stages or zones, a primary stage 71, a secondary stage 72 and a tertiary stage 73, each of the stages having different conditions of temperature and relative humidity maintained by suitable conventional regulating mechanisms (not shown). The dividing partitions are provided with appropriately dimensioned apertures (not shown) to permit continuous movement or passage of the slats from one stage to the next.

In the primary zone 71, the slats are first subjected to drying under predetermined atmospheric conditions, and in order to effect proper drying at this stage-and to condition the slats for subsequent treatment, it has been found that in the primary zone 71, a dry-bulb temperature of F. and a relative humidity of 75% is used with very beneficial results, the particular temperature and relative humidity being adjusted to suit the particular wood being treated, but the figures herein given being used for very wet and dense wood. The slats are kept in the primary zone for about two hours. In the secondary zone 72, a temperature of F. and relative humidity of 56% effects an excellent increase of drying rate, the time being approximately two hours. These conditions are followed in the tertiary zone 73 by a temperature of 160 F. and relative humidity 43%, for about four hours. At the conclusion of the drying station 57, the slat moisture content will average approximately 20 to 50% (based on dry weight).

From the first drying oven 57, the slats are transported still being in the original transverse row arrangement, to a staining station 76 where the slats are impregnated with stain, the stain returning the slats to a high moisture content, for example of the order of 200%.

The stained slats are then dried, in accordance with my method, in a second drying oven 77 having interior partitions 78 and 79 dividing the oven into a first zone 81, a second zone 82 and a third zone 83, each zone resembling the corresponding stages in the first oven but being provided with differing predetermined conditions of drybulb temperature and relative humidity depending on the particular slat material being treated. For wood pencil slats of the hard to dry kind, i. e., wet and dense, it has been found that very satisfactory results are obtained by providing the following atmospheric conditions: in the first zone 81, dry-bulb temperature 120 F., relative humidity 70% for about three hours; in the second zone 82, about four hours at F., 41% for rapid drying; and in the third zone 83, about three hours at F. and 55% relative humidity to bring the slats near to final desired moisture content and to equalize the wetter and dryer slats. It is frequently found advantageous to add a fourth zone 84 for very close control of final desired moisture, the slats in this zone being subjected for about three to four hours at a temperature of 170 F. and 2% relative humidity.

As the slats leave the last zone of the second oven, their moisture content is approximately 8% and the slats are uniformly dry and stress free, in a fully shrunk condition, entirely suitable for their purpose.

The emergent slats, still in their initial arrangement in transverse staggered rows, are then led to a re-sorting station 86 where rejects if any, are divided out, and thence to a packing station 87 and onwardly to storage and subsequent operations such as shipping.

It can therefore be seen that the method of treating slats of my invention not only lends itself admirably to a production line type of operation, but also reduces the amount of in-process inventory, cuts down on slat drying time, and eliminates to a marked extent the number of rejects produced by present day slat drying methods.

What is claimed is:

l. A method of treating wood slats comprising the steps of cutting a plurality of slats to predetermined dimensions, of arranging said slats with their faces in vertical attitude on an elongated perforated supporting surface to form a plurality of rows transversely of said surface, the end portion of the side faces of each of said slats overlapping by a predetermined amount the side face portions of the contiguous slats in the adjacent rows, of subjecting said slats to a first atmosphere of predetermined temperature and relative humidity for a predetermined period of time while said slats are on said surface, of immersing said rows of slats in a stainingtluid bath, and of subjecting said slats in said rows to a second atmosphere of predetermined temperature and relative humidity for a predetermined period of time.

2. A method of treating elongated wood pencil slats comprising the steps of arranging said slats in a plurality of transverse rows on an elongated supporting surface with the faces of said slats normal to said surface, the slats in each of said rows being offset by a predetermined amount from the slats in the adjacent rows, of applying a Wax coating to the exposed faces of said slats, of transporting said rows through a first drying kiln, of immersing said rows in a tank of liquid stain, of transporting said rows through a second drying kiln, and of rearranging said slats into bundles for storage and shipment.

3. A method of preparing wood pencil slat stock including the steps of cutting the stock into a plurality of elongated slats having the grain substantially parallel with the length thereof and having a width several times the thickness, of arranging said slats in a plurality of rows, said slats being edge supported with the grain in substantially horizontal attitude, the ends of said slats in each of said rows being spaced from the ends of the adjacent slats in said each of said rows by a predetermined zu'nount, said rows being alternately staggered whereby the end portions of the faces of each of said slats in each of said rows are overlapped by the adjacent end portions of the faces of each of said slats in the adjacent rows, and of subjecting said slats to a slat-drying atmosphere.

4. A method of treating elongated wood slats comprising the steps of arranging said slats in a plurality of rows with the side faces of said slats in substantially vertical attitude, the adjacent ends of said slats in each of said rows being spaced apart a predetermined distance, the alternate rows of said slats being disposed in predetermined staggered relation whereby the end portions of the side faces of each of said slats are overlapped by the end portions of the side faces of the adjacent slats in the adjacent rows, and subjecting said rows of slats to a slat-drying atmosphere.

5. A method of treating elongated wood slats each having a first side-grain surface, a second side-grain surface greater in extent than said first surface, and an endgrain surface, comprising the steps of positioning said slats in a plurality of alternatingly arranged rows, each of said rows including a plurality of aligned and spaced slats each having said second side'grain surface and said end-grain surface in substantially vertical attitude, each of said alternatingly arranged rows being offset longitudinally from the adjacent rows whereby at least a portion of the adjacent second side-grain surfaces in adjacent rows are in overlapping relation, and of subjecting said slats to a series of controlled predetermined wooddrying atmospheres.

No references cited 

